1. Field of the Invention
The present invention relates to a steering apparatus for varying a steering angle ratio, i.e., a transmission ratio, between an angle through which the steering wheel of a motor vehicle is turned and an angle through which steerable road wheels are steered, based on the yaw angular velocity of the motor vehicle.
2. Description of the Prior Art
Japanese laid-open patent publication No. 62-18366 discloses an automobile steering apparatus having, in a steering force transmitting path for transmitting steering forces from a steering wheel to steerable road wheels, a transmission ratio varying means for varying a transmission ratio, i.e., a ratio of a steered angle of the steerable road wheels to a turned angle of the steering wheel. The transmission ratio is varied by the transmission ratio varying means depending on the speed at which the steering wheel is turned, the speed of the automobile, and the turned angle of the steering wheel.
For improved maneuverability of an automobile when the automobile makes a turn, it is necessary to take into account the following considerations:
FIG. 8 of the accompanying drawings shows the relationship between the steering wheel angle and the lateral acceleration of a motor vehicle while the motor vehicle is making a circular turn having a constant radius. The solid-line characteristic curve represents an understeer response (US) which requires a steering wheel angle to be increased when the lateral acceleration of the motor vehicle is increased as a result of an increased vehicle speed. The dotted-line characteristic curve represents an oversteer response (OS) which requires a steering wheel angle to be reduced when the lateral acceleration of the motor vehicle is increased as a result of an increased vehicle speed. Therefore, insofar as the motor vehicle has an understeer response or an oversteer response, the steering wheel angle should be corrected to an extent depending on the lateral acceleration though the motor vehicle is making a constant circular turn. It is desirable that the steering wheel angle remain substantially constant as indicated by the dot-and-dash-line curve which represents an ideal response.